UK Labour leader Ed Miliband said: "Whatever our disagreements, he always spoke his mind and he has always stood up for what he believed in."

Scottish Labour leader Johann Lamont added: "Alex Salmond turned a minority party into a party of opposition into a party of government and was responsible for there being a referendum on Scotland leaving the United Kingdom.

"He has undoubtedly been an immense figure in Scottish political history."

Ruth Davidson, the leader of the Scottish Conservatives, said Mr Salmond's "decision to step down will help our country come back together again".

She added: "I am pleased that the first minister says he will continue to serve in Scottish politics.

"Scotland will benefit from his experience and service as we move forward."

'Privilege'

Mr Salmond said he would resign as SNP leader at the party's conference in November.

He will then stand down as first minister when the party elects its next leader in a membership ballot.

Ms Sturgeon is seen as a clear frontrunner to replace him.

In her statement, she paid tribute to her "friend, mentor and colleague for more than 20 years".

"He led the SNP into government and has given our country a renewed self confidence," she said.

She added: "Alex's announcement today inevitably raises the question of whether I will be a candidate to succeed him as SNP leader.

"I can think of no greater privilege than to seek to lead the party I joined when I was just 16. However, that decision is not for today."

'Remarkable debate'

Scottish Parliament Presiding Officer Tricia Marwick said: "Members from across the chamber will recognise the first minister as an outstanding parliamentarian and will join me in recognition of his substantial achievements and public service as an MP, MSP and as Scotland's longest-serving first minister.

"He will bow out following the most remarkable national debate and democratic renewal of our nation."

Welsh First Minister Carwyn Jones said Mr Salmond would "leave a large gap in Scottish politics".

"He is an astute politician and was a passionate leader of his party and the Scottish government," Mr Jones added.